1. Field of the Invention
The present invention relates to a communication system, a communication apparatus and a method for controlling the apparatus for communicating large amount of image data by asynchronous TDMA wireless communication.
2. Description of the Related Art
In a wireless data communication system for plural stations to perform asynchronous communication with each other using the same frequency carrier, time division multiple access (TDMA) method is widely used, in which each of the stations performs time division communication. On this occasion, wireless medium access control is performed to access the wireless path.
For example, in a wireless LAN system, plural asynchronous communicating stations can coexist by performing access control using carrier sense multiple access with collision avoidance (CSMA/CA) and back off time in which a medium is selected at random after busy state (hereinafter, referred to a random back off time).
The CSMA/CA procedure confirms the state of the medium by sensing carrier when starting transmission, and sends an electric wave to the wireless medium only when the carrier has not been detected in the medium for a predetermined period. If the carrier is detected, carrier sensing is continued. When the carrier becomes not detected the transmission operation including the carrier sensing is restarted after waiting transmission for a period corresponding to the random back off time.
In addition, the sending station further sends the request to send (RTS) signal, the clear to send (CTS) signal, and the acknowledge (ACK) signal for dealing with so-called hidden terminal problem, in which the other station existing in a location where it cannot sense carrier causes interference in a location of receiving station. Thus, imaginary carrier detection means are realized that normally confirm the response from the other station in data communication.
The above-mentioned system performs collision avoidance by the carrier sensing and the random back off procedure, and further detects collision by the reception confirming response means. However, the system still has high possibility to cause the collision when the hidden terminal exists and the detection is difficult in carrier sensing as mentioned above. It is a problem when the hidden terminal exists that the collision cannot be avoided even if the collision is detected by the reception confirming response means, and the collision causes drop of throughput due to resending procedure after the collision.
In addition, there is another problem that the influence of the overhead may become large if the traffic is not uniform when using RTS/CTS procedure.
An object of the present invention is to solve the above-mentioned problems. The present invention provides a communication system, a communication apparatus and a method for controlling the apparatus, in which plural stations can communicate efficiently by controlling access to the transmission path in accordance with the signal to be sent.
A communication system according to the present invention is for plural communication apparatuses to communicate each other using a single communication path. The system includes first control means for detecting the state of the communication path at transmission of a signal, and for controlling access to the communication path using a back off time selected at random when the communication path is busy, second control means for securing the communication path by sending and receiving a predetermined signal at transmission of the signal, and for controlling the access to the communication path, and communication means for selecting the first or the second control means in accordance with the signal to be sent for communication.
In another aspect, a communication apparatus according to the present invention includes first control means for detecting the state of the communication path, and for controlling access to the communication path using a back off time selected at random when the communication path is busy, second control means for controlling the access to the communication path by sending and receiving the RTS signal and the sending permission signal, and communication means for selecting the first or the second control means in accordance with the signal to be transmitted.
In still another aspect, a method for controlling a communication apparatus according to the present invention is for plural communication apparatuses to communicate each other using a single communication path. The method includes first control step of detecting the state of the communication path, and controlling access to the communication path using a back off time selected at random when the communication path is busy, second control step of controlling the access to the communication path by sending and receiving the RTS signal and the sending permission signal, and third control step of controlling so as to select the first or the second control step for communication in accordance with the signal to be sent.
Other objects of the present invention will be cleared by the detailed description explained below with reference to the accompanied drawings.